Air pollution has long been a high profile public health and environmental issue. Despite success in tackling issues such as acid rain and ozone layer depletion, improving air quality to an acceptable standard is still a major challenge across much of the world.
Although improvements have arisen through technology and environmental legislation, currently there are a number of air pollutants which remain at levels believed to cause significant health, environmental and economic costs.
PM refers to a mixture of microscopic dust and liquid particles suspended in the air. Motor vehicles and combustion processes are a major source, but natural particles include windblown dust, sea salt and pollen. The health impacts of PM relate not only to the particle size distribution, but also with its ability to absorb environmental toxins. PM is classified according to size: PM10 refers to particles under 10 microns in diameter (i.e. 1/100th of millimetre); PM2.5 refers to below 2.5 microns.
Anthropogenic emissions of coarse particles mostly arise from the resuspension of dust by vehicle traffic, construction works and industrial processes. Fine particles mostly originate from combustion processes and atmospheric reactions between other air pollutants.
The smaller particles are believed to be the most harmful to health. It is estimated that PM pollution causes an average 7-8 months reduction in life expectancy per person in the UK. The overall evidence indicates a causal relationship between PM2.5 exposure and health impacts from cardiovascular and respiratory diseases.
NOx is composed mostly of nitric oxide (NO) and nitrogen dioxide (NO2). These are formed naturally, but within developed countries vehicular traffic will typically contribute at least 50%. As a by-product of combustion, concentrations vary steeply with distance to vehicle traffic. It is therefore frequently used as an indicator of urban air quality.
As an irritant gas, adverse health effects mostly relate to the aggravation of respiratory diseases. An increase in emergency department visits and mortality among people with such conditions is associated with high NOx levels.
Ozone is a ‘secondary pollutant, formed by complex photochemical processes in the lower atmosphere, involving NOx and volatile organic compounds (VOCs) as precursors
As its formation is driven by sunlight, levels are highly seasonal. As a powerful oxidant and respiratory irritant, the public health impact of excessive ozone levels largely relates to its effects on people with respiratory conditions such as asthma and COPD. Ozone pollution also has detrimental effects on natural vegetation and agriculture.